Sheet stacking apparatus and image forming apparatus

ABSTRACT

A sheet stacking apparatus has a stack tray which can move so that a sheet stack surface keeps a constant position from a discharge outlet and which is a finisher capable of using the stack tray by switching it. When stacking the maximum number of sheets to be stacked on the stack tray and preventing stacking of sheets on the stack tray up to the maximum number of sheets to be stacked and the number of sheets stacked on the stack tray reaches a predetermined number of sheets to be stacked which does not prevent up to the maximum number of sheets from being stacked on the stack tray, stacking of sheets on the stack tray is temporarily prohibited to stack sheets by switching to the stack tray. Thereby, all sheet stack trays can stack up to the maximum number of sheets to be stacked, decrease of the number of stacked sheets is prevented in the whole sheet stacking apparatus and the number of temporal interruptions of a job is decreased.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a sheet stacking apparatus for stacking sheets output from an image forming apparatus such as a copying machine, printer or facsimile, particularly to a sheet stacking apparatus so that all stack trays can stack the maximum number of sheets to be stacked to prevent the number of sheets to be stacked of the whole apparatus and decrease the number of temporary interrupt times of jobs and an image forming apparatus having the sheet stacking apparatus.

2. Related Background Art

A sheet processing apparatus as a sheet stacking apparatus is conventionally proposed which successively conveys sheets in which an image is recorded by an image forming apparatus into the sheet processing apparatus and selectively applies predetermined processing such as punch processing, stapling or sorting. Some of the sheet processing apparatus respectively have a plurality of sheet stack trays (stack trays) capable of performing vertical movement. The sheet stack tray is movement-controlled so that an uppermost surface of the sheets stacked on the stack tray keeps a constant position from a discharge outlet for discharging sheets. Moreover, a user sets a sheet stack tray to be used every job such as copying, FAX or printing and can switch sheet stack trays when the job is executed.

Furthermore, it is disclosed that, when sheets are stacked up to the maximum number of sheets to be stacked on a sheet stack tray, the stacking of the left sheets is prohibited and the left sheets are stacked on another sheet stack tray (refer to Japanese Patent Application Laid-Open Nos. H10-152260 and H10-305958). According to this sheet processing apparatus, it is possible to continuously discharge sheets to a switched sheet stack tray without interrupting the image forming job.

However, in the above case, for example, a sheet processing apparatus as a sheet stacking apparatus having two sheet stack trays capable of vertical movement sometimes cannot stack sheets up to the maximum number of sheets to be stacked on an upper sheet stack tray after stacking sheets up to the maximum number of sheets to be stacked on a lower sheet stack tray. This occurs in a case in which the moving range of the upper sheet stack tray to be moved so that the uppermost surface of the sheets stacked on the stack tray keeps a constant position from a sheet discharge outlet is controlled by sheets stacked on the lower sheet stack tray. In this case, the upper sheet stack tray is controlled in movement before stacking sheets up to the maximum number of stacking sheets as described above. Therefore, the number of sheets to be stacked in a sheet processing apparatus is decreased. Moreover, when movement of a sheet stack tray is controlled before one job is completed as described above, it is necessary to temporarily interrupt an image forming job until a user removes sheets from the sheet stack tray. Furthermore, as described above, when the number of sheets to be stacked is decreased, the number of times the job is interrupted increases.

SUMMARY OF THE INVENTION

An object of the present invention is to make it possible that all sheet stack trays can stack sheets up to the maximum number of sheets to be stacked, prevent the number of sheets to be stacked from decreasing in the whole sheet processing apparatus and decrease the temporary number of interrupt times.

Another object of the present invention is to be possible to stack sheets up to the maximum number of sheets to be stacked on all sheet stack trays of a sheet processing apparatus. Therefore, it is possible to prevent the number of sheets to be stacked in the whole sheet processing apparatus from decreasing. Moreover, because it is possible to decrease the number of temporary interrupt times of jobs, the sheet processing efficiency is not deteriorated.

A further object of the present invention is to provide a sheet stacking apparatus capable of stacking sheets using a plurality of sheet stack trays by switching with each other, including a plurality of sheet stack trays each of which moves to keep an uppermost surface of sheets stacked on the sheet stack tray in a constant position from a sheet discharge outlet when sheets are discharged on each of said plurality of sheet stack trays, wherein said plurality of sheet stack trays include: a first sheet stack tray, when the maximum number of sheets which are allowed to be stacked on said first sheet stack tray have been stacked thereon, which is which prevented from stacking on another sheet stack tray at least until another maximum number of sheets which are allowed to be stacked on another sheet stack tray are stacked; and a second sheet stack tray, when the maximum number of sheets which are allowed to be stacked on said second sheet stack tray have been stacked thereon, which is not prevented from stacking on another sheet stack tray at least until another maximum number of sheets which are allowed to be stacked on another sheet stack tray are stacked, wherein in a case where the number of sheets stacked on the first sheet stack tray reaches a predetermined number of sheets to be stacked which does not prevent stacking of sheets onto said another sheet stack trays until the number of sheets stacked on said another sheet stack trays reach the maximum number of sheets stacked, stacking of sheets on the first sheet stack tray is temporarily prohibited to stack sheets by switching the present sheet stack tray to a sheet stack tray other than the first sheet stack tray reaching the predetermined number of sheets.

A further object of the present invention to provide an image forming apparatus including an image forming part which forms an image on a sheet; and a sheet stacking apparatus which stacks a sheet on which an image is formed by the sheet forming part, wherein said sheet stacking apparatus capable of using a plurality of sheet stack trays by switching with each other, wherein said sheet stacking apparatus includes a plurality of sheet stack trays each of which moves to keep an uppermost surface of sheets stacked on the sheet stack tray in a constant position from a sheet discharge outlet when sheets are discharged on each of said plurality of sheet stack trays, wherein said plurality of sheet stack trays includes: a first sheet stack tray, when the maximum number of sheets which are allowed to be stacked on said first sheet stack tray have been stacked thereon, which is prevented from stacking on another sheet stack tray at least until another maximum number of sheets which are allowed to be stacked on another sheet stack tray are stacked; and a second sheet stack tray, when the maximum number of sheets which are allowed to be stacked on said second sheet stack tray have been stacked thereon, which is not prevented from stacking on another sheet stack tray at least until another maximum number of sheets which are allowed to be stacked on another sheet stack tray are stacked, wherein in a case where the number of sheets stacked on the first sheet stack tray reaches a predetermined number of sheets to be stacked which does not prevent stacking of sheets onto said another sheet stack trays until the number of sheets stacked on said another sheet stack trays reach the maximum number of sheets stacked, stacking of sheets on the first sheet stack tray is temporarily prohibited to stack sheets by switching the present sheet stack tray to a sheet stack tray other than the first sheet stack tray reaching the predetermined number of sheets.

A still further object of the present invention will become more apparent by referring to the following detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view showing a general configuration of an image forming apparatus of an embodiment of the present invention;

FIG. 2 is a block diagram showing a configuration of a controller for controlling the whole of an image forming apparatus;

FIG. 3 is a schematic sectional view of an image forming apparatus showing a flow of a sheet for face-up discharge;

FIG. 4 is a schematic sectional view of an image forming apparatus showing a flow of a sheet when processed;

FIG. 5 is a sectional view of an essential part showing a configuration of a stack tray of a sheet processing apparatus;

FIGS. 6A, 6B, 6C and 6D are illustrations of the switching operation of a stack tray due to occurrence of stack-over;

FIG. 7 is an illustration of a state of a stack tray when sheets of the maximum number of sheets are stacked;

FIG. 8 is an illustration of a state of stack trays when a stack limit occurs;

FIGS. 9A, 9B, 9C, 9D, 9E and 9F are operational illustrations in which two tack trays respectively stack up to the maximum number of sheets;

FIG. 10 is a flowchart for switching stack trays in which two stack trays respectively stack up to the maximum number of sheets.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereafter, by referring to the accompanying drawings, preferred embodiments of the present invention are illustratively described in detail. An embodiment of an image forming apparatus having a sheet processing apparatus is described below. In this case, a copying machine is shown as an image forming apparatus and a finisher is shown as a sheet processing apparatus.

First, a schematic configuration of the whole of an image forming apparatus is described by referring to FIG. 1. FIG. 1 is a sectional view showing an internal structure of the whole of an image forming apparatus which is an embodiment of the present invention.

As shown in FIG. 1, an image forming apparatus 10 has a printer 100 serving as an image forming part and an image reader 200 serving as an image reading part. Moreover, the image forming apparatus 10 has a document feeder 400, finisher 500 serving as a sheet stacking apparatus and operation display part 600.

The document feeder 400 is mounted on the image reader 200. The document feeder 400 separates and feeds originals set on an original tray upwards one by one from the top page in the left direction of the figure in order. Then, the document feeder 400 stops the originals at a predetermined position on a platen glass 202 through a curved path. Under this state, images of the originals are read by moving a scanner unit 201 from left to right for scanning. When the scanner unit 201 performs scanning, the reading surface of an original is irradiated with the light of a lamp of the scanner unit 201 and the light reflected from the original is led to a lens through a mirror. The light passing through the lens is imaged on the image pickup surface of an image sensor 203. The optically-read image is converted into image data by the image sensor 203 and output. Predetermined processing is applied to the image data output from the image sensor 203 by an image signal control part 281 to be described later (refer to FIG. 2) and then the image data is input to the exposure control part 101 of the printer 100 as a video signal.

Then, a case of forming an image on either side of a sheet is described by referring to FIG. 3. The exposure control part 101 of the printer 100 modulates a laser beam in accordance with the input video signal and outputs the laser beam. This laser beam is irradiated onto a photosensitive drum 102 while being scanned by a not-illustrated polygon mirror or the like. An electrostatic latent image corresponding to the scanned laser beam is formed on the photosensitive drum 102. The electrostatic latent image on the photosensitive drum 102 is visualized as a developer image by a developer supplied from a development counter 103.

Sheets are selectively supplied from a cassette 111 or 112 or manual feed tray 113. These sheets are led to a conveying path and once stopped by the front end of each sheet being anti-climbed against a registration roller 114. Thereafter, the sheets are conveyed between the photosensitive drum 102 and a transfer part 104 at the timing synchronous with start of irradiation of the laser beam. The developer image formed on the photosensitive drum 102 is transferred onto a sheet supplied by the transfer part 104. Moreover, because the sheets are once stopped by the front end of each sheet being anti-climbed against the registration roller 114, a tilt of each sheet is corrected.

A sheet on which the developer image is transferred is conveyed to a fixing portion 105, which pressurizes the sheet by heat to fix the developer image on the sheet. The sheet passing through the fixing portion 105 is brought out from the printer 100 to the finisher 500 through a discharge roller 116 by a flapper 118. In this case, the sheet is brought out in a face-up state in which the image surface of the sheet is turned upwards.

To bring out the sheet in a face-down state in which the image surface is turned downwards, the sheet is brought to a sheet reverse conveying path 119 by a conveying roller 115 by the printer switching a flapper 118 when the sheet passes through the fixing portion 105. Then, by the printer switching back the sheet, the surface of the sheet is reversed and discharged from the printer 100 to the finisher 500 through the discharge roller 116.

The sheet discharged from the printer 100 is sent to the finisher 500. The finisher 500 can selectively apply stapling or sorting to bundle of sheets. In the case of this processing, it is possible to select or cancel the staple mode or sorting mode by the operation display part 600. When sorting or stapling is not set and the sheet is directly discharged, the flapper 518 is switched and the sheet is discharged to a stack tray 510 by a conveying roller 517 via a non-sorting path 516.

Then, conveying of a sheet when sorting or stapling is set to a sheet is described below by referring to FIG. 4. In the case of a sheet to be processed, the surface and back of the sheet are reversed by the switching back conveyance and the sheet is discharged from the printer 100 to the finisher 500 in a face-down state. To process the sheet, the sheet is discharged from discharge rollers 501 and 502 of the finisher 500 onto a bundle discharge belt 503. A intermediate-processing tray 508 having a low friction is set to a position several millimeters higher than the bundle discharge belt 503 in parallel with the belt 503 and for accurately speaking the sheet is discharged to the intermediate-processing tray 508. The discharged sheet drops downwards in the right direction of the figure along the slantly set intermediate-processing tray 508 (bundle discharge belt 503) by its dead weight. Moreover, when a sector return roller 504 rotates counterclockwise, a friction member set to the return roller 504 like a circular arc contacts with the sheet, drops the sheet in the right downward direction and makes an end of the sheet anti-climb a stopper plate 507. Thereby, arrangement of sheets in the longitudinal direction (feed direction) is performed. Moreover, a matching plate 506 is set to this side and recessed side on the intermediate processing tray 508 and driven whenever the sheet is discharged onto the intermediate processing tray 508 and transverse-directional (width-directional) arrangement is applied to the sheets on the intermediate processing tray 508. Furthermore, when a predetermined number of sheets is discharged and stacked onto the intermediate processing tray 508, the bundle discharge belt 503 is driven and the sheets are discharged onto a stack tray 510 or 511 serving as a sheet stack tray.

Furthermore, when the staple mode is set to the operation display part 600, sheets forming a sheet bundle to be stapled are discharged onto the intermediate processing tray 508, the sheets are arranged by the return roller 504 and matching plate 506 and then the staple operation is performed by driving a stapler 505. The stapled sheet bundle is discharged onto the stack tray 510 or 511 by the bundle discharge belt 503.

Furthermore, the stapler 505 is movable in the transverse direction (width direction) from the sheets on the intermediate processing tray 508 and perform the staple operation at optional positions of this side and recessed side. A position for applying stapling to a sheet is set by the operation display part 600.

Then, a configuration of a controller for controlling the whole of the image forming apparatus is described below by referring to FIG. 2. FIG. 2 is a block diagram showing a configuration of the controller for controlling the whole of the image forming apparatus in FIG. 1.

As shown in FIG. 2, the controller has a CPU circuit part 150 and the CPU circuit part 150 includes a CPU (not illustrated), ROM 151 and RAM 152. Then, blocks 480, 280, 281, 282, 283, 180, 680 and 580 are generally controlled by a control program stored in the ROM 151. Moreover, the RAM 152 temporarily holds control data and moreover, used as an operation area for the processing for control.

The document feeder control part 480 drives the document feeder 400 in accordance with a designation from the CPU circuit part 150. The image reader control part 280 drives the above scanner unit 202 and image sensor 203 and transfers an analog image signal output from the image sensor 203 to the image signal control part 281.

The image signal control part 281 applies various processes after converting an analog image signal from the image sensor 203 into a digital signal, converts the digital signal into a video signal and outputs the video signal to the printer control part 180. The image signal control part 281 also applies various processes to the digital image signal input from the computer 283 through the external I/F 282, converts the digital image signal into a video signal and outputs the video signal to a printer control part 180. The processing by the image signal control part 281 is controlled by the CPU circuit part 150. The printer control part 180 drives the above exposure control part 101 in accordance with the input video signal.

The operation-display control part 680 exchanges information between the operation display part 600 and the CPU circuit part 150. The operation display part 600 has a plurality of keys for setting various functions on image formation and a display part for displaying the information showing a set state. Then, the operation display part 600 outputs a key signal corresponding to the operation of each key to the CPU circuit part 150 and displays corresponding information on the display part in accordance with a signal from the CPU circuit part 150.

The finisher control part 580 is mounted on the finisher 500 to drive the whole of the finisher by exchanging information with the CPU circuit part 150.

Then, operations of stack trays in the finisher are described below by referring to FIG. 5. The stack trays 510 and 511 are sheet stack trays for stacking the sheets discharged from the intermediate processing tray 508 of the finisher 500. The stack trays 510 and 511 respectively perform vertical movement by using a not-illustrated motor as a driving source. Moreover, sheet surface detection sensors 514 and 515 having a not-illustrated light emitting part and light receiving part respectively detect the uppermost surface of the sheets on the stack trays 510 and 511. In accordance with inputs from the sheet surface detection sensors 514 and 515, the stack trays 510 and 511 vertically move stack trays in accordance with inputs from the sheet surface detection sensors 514 and 515 to keep the uppermost surface position (sheet stack surface) to the sheet discharge outlet to which sheets are discharged. Moreover, sheet presence/absence detection sensors 512 and 513 detect presence or absence of sheets on the stack trays 510 and 511. Furthermore, the number of sheets to be stacked on each of the stack trays 510 and 511 is obtained by counting the number of sheets to be discharged to each of the stack trays 510 and 511. Furthermore, when the number of sheets to be discharged reaches the maximum number of sheets to be stacked on each of the stack trays 510 and 511, stacking of sheets on stack trays is prohibited. The maximum number of sheets to be stacked on the stack trays depends on the type or size of a sheet. Therefore, for example, when using one A4-size plain sheet, weighting is performed in accordance with the type or size of sheets stacked on a stack tray by assuming one A3-size plain sheet as 1 and one A4-size cardboard as 1.5 to count the number of sheets to be discharged. In the case of this embodiment, the maximum number of sheets to be stacked on a stack tray is obtained by counting and weighting the number of sheets to be discharged. However, it is also allowed to obtain the position of a stack tray and the position of a sheet stack surface by detecting them directly by a sensor.

Under the normal waiting state, the sheet stack surface of the stack tray 510 is located at the discharge outlet of a non-sort path 516 and the sheet stack surface of the stack tray 511 is located at the discharge outlet of the intermediate processing tray 508. In the case of default setting, a non-sort job is discharged to the stack tray 510 and a job for performing stapling or sorting is discharged to the stack tray 511.

It is possible to use the stack trays 510 and 511 by switching them. Switching of the stack trays includes a case of switching stack trays when reaching the maximum number of sheets to be stacked of a stack tray when discharging sheets and a case of switching stack trays by setting a job every mode.

First, switching of tack trays when sheets are stacked up to the maximum number of sheets to be stacked of a stack tray is described below by referring to FIG. 6.

First, as shown in FIG. 6A, sheet bundles are stacked on the stack tray 510. Continuously, sheet bundles are stacked on the stack tray 510. When the number of sheets to be stacked reaches the maximum number of sheets which are allowed to be stacked on the stack tray 510, stacking of sheets on the stack tray 510 is prohibited and an image forming job in the image forming apparatus is interrupted.

In this case, when the maximum number of sheets which are allowed to be stacked on the stack tray 510 is reached while sheets forming a sheet bundle are discharged and switching of stack trays is necessary, the image forming job is interrupted after stacking of all sheets forming a sheet bundle is completed to switch stack trays. This is because when switching stack trays while discharging sheets forming a sheet bundle, a sheet bundle is stacked on two stack trays. Therefore, as described above, the image forming job is interrupted after the image forming apparatus discharges up to the final page of a sheet bundle to start switching of stack trays.

However, the number of sheets which can be stacked by exceeding the maximum number of sheets to be stacked of the stack tray 510 as a margin is limited. When it is determined that stacking of sheets by exceeding the maximum number of sheets to be stacked of the stack tray 510 is impossible, the image forming job is interrupted, removing of sheets from the tack tray 510 by informing means such as the operation display part 600 is urged to a user, it is confirmed by the sheet presence/absence detection sensor 512 that sheets on the stack tray 510 are gone out and then the image forming job is restarted.

Moreover, it is allowed to calculate the scheduled number of sheets to be stacked on the stack tray 510 every sheet bundle from job set content, compare the number of sheets to be stacked for a unit of sheet bundle with the maximum number of sheets to be stacked on the stack tray 510, interrupt the image forming job when discharge of sheet bundle one before a sheet bundle when it is determined that the maximum number of sheets to be stacked is exceeded, urge the user to remove sheets from the stack tray 510 by the informing means such as the operation display part 600, confirm by the sheet presence/absence detection sensor 512 that the sheets on the stack tray 510 are gone out and restart the image forming job.

Then, as shown in FIG. 6, when stacking of sheets on the stack tray 510 is prohibited but stacking of sheets on the stack tray 511 is not prohibited, the present state is switched to discharging to the stack tray 511. Therefore, rise of the stack trays 510 and 511 is started. The stack tray 501 is moved up to the evacuation position above the discharge outlet of the intermediate processing tray 508 and the sheet stack surface of the stack tray 511 is moved up to the discharge outlet of the intermediate processing tray 508 in accordance with an output of the sheet surface detection sensor 514. When the stack tray 510 rises in accordance with the stack-tray switching operation, a not-illustrated shutter closes the discharge outlet of the intermediate processing tray 508 to prevent the sheets stacked on the stack tray 510 from backing up from the discharge outlet of the intermediate processing tray 508.

As shown in FIG. 6C, when the sheet stack surface of the stack tray 511 rises up to the vicinity of the discharge outlet of the intermediate processing tray 508, the image forming job is restarted. Then, as shown in FIG. 6D, stack of processed sheets is discharged from the intermediate processing tray and sheets are stacked on the stack tray 511.

When the stack trays 510 and 511 respectively reach the maximum number of sheets which are allowed to be stacked on each of the trays and stacking of sheets is prohibited, the image forming job is interrupted and it is communicated by the operation display part 600 to a user to remove sheets from the stack trays. When removal of sheets from the stack trays is detected by the sheet presence/absence detection sensors 512 and 513, the image forming job is restarted. The operation after restarting the image forming job means that the operation described above by referring to FIG. 6 is repeated up to completion of the image forming job.

Then, a case in which stack trays are switched in accordance with setting every job mode is described. When switching stack trays in accordance with the setting every job mode, in order that an output of a copy job is discharged to the stack tray 511 and an output of a printer job or a FAX job is discharged to the stack tray 510, a user can previously perform the setting of a stack tray to which an output is discharged every job mode, by the operation display part 600. When a stack tray set every job mode is different from a stack tray at the discharge outlet in performing a job, the job stack tray is vertically moved and stack trays are switched.

FIG. 7 shows states of a stack tray and stacked sheets when stacking the maximum number of sheets is performed on the stack trays 510 and 511 in the sheet processing apparatus. However, as shown in FIG. 8, when the maximum number of sheets which are allowed to be stacked on the stack tray 511 have been stacked on the stack tray 511 before the maximum number of sheets which are allowed to be stacked on the stack tray 510 are stacked on the stack tray 510, downward movement of the stack tray 510 is restricted at the discharge outlet of the intermediate processing tray 508. Therefore, when stacking a predetermined number of sheets or more on the stack tray 511, it is impossible to stack sheets on the stack tray 510 until the maximum number of sheets which are allowed to be stacked on the stack tray 510 for a job discharged through the intermediate processing tray are stacked on the stack tray 510. Thereby, the number of sheets which are allowed to be stacked on the stack tray 510 decreases and the number of sheets to be stacked of the sheet stacking apparatus decreases.

Therefore, in the case of this embodiment, when stacking the maximum number of sheets to be stacked on the stack tray 511 serving as a first sheet stack tray of two stack trays and preventing the maximum number of sheets. from being stacked on the stack tray 510 serving as a second sheet stack tray, sheets are stacked by switching these two stack trays as described below. When the number of sheets to be stacked on the stack tray 511 stacking the sheets discharged from the discharge outlet of the intermediate processing tray reaches a predetermined quantity which does not prevent stacking of a maximum number of sheets to be stacked on the stack tray 510, stacking of sheets on the stack tray 511 having reached the predetermined quality is temporarily prohibited and sheets are stacked by switching the stack tray 511 to the stack tray 510.

That is, in the case of this embodiment, the stack tray 511 is switched to the stack tray 510 before sheets are stacked on the stack tray 511 until a limit in which the stack restriction on the stack tray 510 occurs to stack up to the maximum number of sheets to be stacked on the stack tray 510.

Moreover, in the case of this embodiment, even if the maximum number of sheets to be stacked is stacked on the switched stack tray 510, when the stack tray 510 does not prevent stacking of sheets up to the maximum number of sheets to be stacked on the stack tray 511 which is a sheet stack tray other than the stack tray 510, sheets are stacked as described below by switching these two stack trays. When the number of sheets to be stacked on the switched stack tray 510 reaches the maximum number of sheets, stacking of sheets on the switched stack tray 510 is prohibited and sheets are stacked by switching the stack tray 510 to the stack tray 511 which is a sheet track tray other than the stack tray 510. Thereby, the stack trays 510 and 511 can respectively stack up to the maximum number of sheets to be stacked.

Hereafter, switching of the stack trays 510 and 511 is described in detail. In this case, operations when stacking sheets up to the maximum number of sheets on each of the stack trays 510 and 511 are described for a job in which processing such as stapling or sorting is set by referring to FIGS. 9A to 9F and FIG. 10. FIGS. 9A to 9F are operational illustrations for stacking sheets up to the maximum number of sheets on each of the stack trays 510 and 511. FIG. 10 is a flowchart of tray switching control when stacking sheets up to the maximum number of sheets to be stacked.

When a job for the processing followed by stapling or sorting is executed and sheets are discharged from the image forming apparatus to the finisher, a sheet bundle is stacked on the stack tray 511 depending on the initial state as shown in FIG. 9A (S1002). At this point of time, by stacking a sheet bundle by switching to the stack tray 510, a sheet bundle can be stacked up to the maximum number of sheets to be stacked of the whole of the finisher. However, when the number of originals or numeral of a job is small, sheets can be stacked on only the stack tray 511. Moreover, when switching stack trays at this point of time, First Copy Output Time FCOT may occur or productivity may lower. Therefore, switching of stack trays is not performed at this point of time.

Then, sheets are stacked on the stack tray 510 up to a predetermined number of sheets (predetermined quantity) which does not prevent stacking of sheets on the stack tray 510 up to the maximum number of sheets to be stacked (S1003). In this case, when a predetermined number of sheets is stacked on the stack tray 511, it is determined in S1004 whether sheets for forming a sheet bundle are currently discharged. That is, when sheets are stacked on the stack tray 511 up to a predetermined number of sheets and sheets for forming a sheet bundle are currently discharged, stacking of sheets is continued up to a break between stacks of sheets (S1005). However, when sheets are stacked up to the break between stacks of sheets, the image forming job is interrupted (S1006) and stacking of sheets on the stack tray 511 is temporarily prohibited (S1007).

Then, as shown in FIG. 9B, lowering is started with the stack trays 510 and 511 (S1008). The stack tray 511 is evacuated up to the lowest point to which the stack tray 511 can move to lower the sheet stack surface of the stack tray 510 up to the discharge outlet of the intermediate processing tray 508 in accordance with an output of the sheet surface detection sensor 514. When the sheet stack surface of the stack tray 510 lowers up to the discharge outlet of the intermediate processing tray 508, the mage forming job which has been interrupted is restated (S1009), stacks of sheets are discharged to the stack tray 510 as shown in FIG. 9C and stacked (S1010).

Then, stacking is continued until the number of sheets to be stacked on the stack tray 510 reaches the maximum number of sheets (S1011). In this case, when the maximum number of sheets to be stacked is stacked on the stack tray 510, it is determined in S1012 whether sheets forming a sheet bundle are currently discharged. That is, when sheets are stacked on the stack tray 510 up to the maximum number of sheets to be stacked and sheets forming a sheet bundle are currently discharged, stacking of sheets is continued up to a break between stacks of sheets (S1013). However, when sheets are stacked up to a break between stacks of sheets, the image forming job is interrupted (S1014) to prohibit stacking of sheets on the stack tray 510 (S1015) as shown in FIG. 9D.

Then, stack prohibition to the stack tray 511 is cancelled (S1016) and the stack trays 510 and 511 are raised as shown in FIG. 9E (S1017). The stack tray 510 is moved to the evacuation position above the discharge outlet of the intermediate processing tray 508 and the sheet stack surface of the stack tray 511 is raised up to the discharge outlet of the intermediate processing tray 508 in accordance with an output of the sheet surface detection sensor 514. When the sheet stack surface of the stack tray 511 is raised up to the discharge outlet of the intermediate processing tray 508 and stopped, the image forming job which has been interrupted is restarted (S1018) to start stacking of sheets on the stack tray 511 as shown in FIG. 9F (S1019).

Because the maximum number of sheets to be stacked is stacked on the stack tray 510 as described above, it is possible to stack up to the maximum number of sheets on the switched stack tray 511. Thereby, even when the processing such as stapling or sorting is set, it is possible to stack up to the maximum number of sheets on both the stack trays 510 and 511 as shown in FIG. 7.

As described above, according to this embodiment, it is possible to stack up to the maximum number of sheets on all stack trays of the sheet processing apparatus. Therefore, it is possible to prevent the number of sheets to be stacked on the sheet processing apparatus from decreasing and decrease the number of temporal interruptions of a job.

In the case of the above-described embodiment, a case of two stack trays is described. However, the present invention is not restricted to this case. The same advantage can be obtained by applying the present invention to a case of three or more stack trays. For example, when there is only one stack tray among three or more stack trays for preventing up to the maximum number of sheets from being stacked on other two stack trays, it is possible to stack up to the maximum number of sheets on all stack trays of the sheet processing apparatus by applying the present invention.

In the case of the above-described embodiment, a copying machine is described as an image forming apparatus. However, the present invention is not restricted to the above case. For example, it is also possible to use another image forming apparatus such as one of a printer and facsimile system or composite system obtained by combining these functions. The same advantage can be obtained by applying the present invention to a sheet processing apparatus as a sheet stacking apparatus used for the image forming apparatus.

Moreover, in the case of the above-described embodiment, a sheet processing apparatus as a sheet stacking apparatus is described which can be set to or removed from the image forming apparatus. However, the present invention is not restricted to the above case. It is also allowed to use a sheet processing apparatus as a sheet stacking apparatus integrated with the image forming apparatus. By applying the present invention to the sheet processing apparatus, the same advantage can be obtained.

This application claims priority from Japanese Patent Application No. 2005-140708 filed on May 13, 2005, which is hereby incorporated by reference herein. 

1. A sheet stacking apparatus capable of stacking sheets using a plurality of sheet stack trays by switching with each other, comprising: a plurality of sheet stack trays each of which moves to keep an uppermost surface of sheets stacked on the sheet stack tray in a constant position from a sheet discharge outlet when sheets are discharged on each of said plurality of sheet stack trays, wherein said plurality of sheet stack trays include: a first sheet stack tray, when the maximum number of sheets which are allowed to be stacked on said first sheet stack tray have been stacked thereon, which is which prevented from stacking on another sheet stack tray at least until another maximum number of sheets which are allowed to be stacked on another sheet stack tray are stacked; and a second sheet stack tray, when the maximum number of sheets which are allowed to be stacked on said second sheet stack tray have been stacked thereon, which is not prevented from stacking on another sheet stack tray at least until another maximum number of sheets which are allowed to be stacked on another sheet stack tray are stacked, wherein in a case where the number of sheets stacked on the first sheet stack tray reaches a predetermined number of sheets to be stacked which does not prevent stacking of sheets onto said another sheet stack trays until the number of sheets stacked on said another sheet stack trays reach the maximum number of sheets stacked, stacking of sheets on the first sheet stack tray is temporarily prohibited to stack sheets by switching the present sheet stack tray to a sheet stack tray other than the first sheet stack tray reaching the predetermined number of sheets.
 2. A sheet stacking apparatus according to claim 1, wherein when the switched sheet stack tray is the second sheet stack tray and the number of sheets of the second sheet stack tray reaches the maximum number of sheets to be stacked, stacking of sheets on the switched sheet stack tray is prohibited to stack sheets by switching the present stack tray to another sheet stack tray.
 3. A sheet stacking apparatus according to claim 1, wherein when the switched sheet stack tray is the first sheet stack tray and the number of sheets stacked on the switched sheet stack tray reaches the predetermined number of sheets to be stacked, stacking of sheets on the switched sheet stack tray is prohibited to stack sheets by switching the present stack tray to another sheet stack tray.
 4. A sheet stacking apparatus according to claim 2, wherein when it is necessary to switch the sheet stack tray while discharging sheets for forming a sheet bundle, the sheet stack tray is switched after stacking of all sheets for respectively forming the sheet bundle is completed.
 5. An image forming apparatus comprising: an image forming part which forms an image on a sheet; and a sheet stacking apparatus which stacks a sheet on which an image is formed by the sheet forming part, wherein said sheet stacking apparatus capable of using a plurality of sheet stack trays by switching with each other, wherein said sheet stacking apparatus comprises: a plurality of sheet stack trays each of which moves to keep an uppermost surface of sheets stacked on the sheet stack tray in a constant position from a sheet discharge outlet when sheets are discharged on each of said plurality of sheet stack trays, wherein said plurality of sheet stack trays includes: a first sheet stack tray, when the maximum number of sheets which are allowed to be stacked on said first sheet stack tray have been stacked thereon, which is prevented from stacking on another sheet stack tray at least until another maximum number of sheets which are allowed to be stacked on another sheet stack tray are stacked; and a second sheet stack tray, when the maximum number of sheets which are allowed to be stacked on said second sheet stack tray have been stacked thereon, which is not prevented from stacking on another sheet stack tray at least until another maximum number of sheets which are allowed to be stacked on another sheet stack tray are stacked, wherein in a case where the number of sheets stacked on the first sheet stack tray reaches a predetermined number of sheets to be stacked which does not prevent stacking of sheets onto said another sheet stack trays until the number of sheets stacked on said another sheet stack trays reach the maximum number of sheets stacked, stacking of sheets on the first sheet stack tray is temporarily prohibited to stack sheets by switching the present sheet stack tray to a sheet stack tray other than the first sheet stack tray reaching the predetermined number of sheets.
 6. An image forming apparatus according to claim 5, wherein when the switched sheet stack tray is the second sheet stack tray and the number of sheets stacked on the switched sheet stack tray reaches the maximum number of sheets to be stacked, stacking of sheets on the switched sheet stack tray is prohibited to stack sheets by switching the present stack tray to another sheet stack tray.
 7. An image forming apparatus according to claim 5, wherein when the switched sheet stack tray is the first sheet stack tray and the number of sheets to be stacked on the switched sheet stack tray reaches the predetermined number of sheets to be stacked, stacking of sheets on the switched first sheet stack tray is prohibited to stack sheets by switching the present stack tray to another sheet stack tray.
 8. An image forming apparatus according to claim 6, wherein when it is necessary to switch the sheet stack tray while discharging sheets for forming a sheet bundle, the sheet stack tray is switched after stacking of all sheets for forming the sheet bundle is completed. 